Silicon carbide filers have many uses such as a reinforcement material for ceramic matrix composites. For example, silicon carbide (SiC) reinfored silicon nitride (Si.sub.3 N.sub.4) powder can be used for the manufacture of turbine blades. Before firing, the well known process of slip casting can be used to produce a SiC fiber/Si.sub.3 N.sub.4 powder compact in the form of the turbine blade. Slip casting, used for many years to produce single component powder compacts, involves the dispersion of the powder in a liquid, typically water. It has been found that the slip casting process is dependent on the surface chemistry and composition of the powders. When a particular powder does not disperse well in slip casting medium, a modification--such as changing the pH or adding a dispersant--can often be made which will allow proper dispersion. In a SiC fiber reinforced ceramic the situation is more complicated as two very different surfaces are present. A further complication is that SiC fibers available commercially can have dramatically different surfaces. Thus, if processing conditions are developed for one type of fiber with a particular surface composition, a suitable product may not be produced if a fiber with a different surface composition is used.
There are other advantages to using fibers with a consistent surface composition. For example, the surface composition of SiC fibers (or other types of fibers) affects the reactions and/or the reaction kinetics between the fibers and the matrix. It has been demonstrated that use of SiC fibers from different sources can result in significant difference in fracture toughness values in SiC fiber inforced Al.sub.2 O.sub.3. Recently, Tiegs and coworkers in "Interfaces in Alumnina-SiC Fiber Composites", Ceramic Microstructures 86: The Role of Interfaces, J. Pask and A. G. Evans, editors, Material Sciences Res. Ser. #21, pages 911-918 (1987), reported experimental results which indicated that surface silica content was a controlling factor for the SiC fiber reinforced Al.sub.2 O.sub.3 composite fracture toughness. The higher toughness is attributed to lower surface SiO.sub.2 (silica) content, probably due to the lower reactivity between fiber and matrix than for fibers with high SiO.sub.2 surface content.
One of the basic problems in the development of SiC fiber reinforced Si.sub.3 N.sub.4 composites is chemical attack upon SiC fibers by sintering aids, starting powder or fiber impurities, and/or the sintering atmosphere. One of the main agents causing such attack is SiO.sub.2. We have performed thermodynamic calculations that indicate that the presence of carbon will suppress SiC degradation by means of SiO.sub.2 attack. A more consistent surface compostion would result in more uniform reactions during processing and thus a more uniform microstructure in the final product. Additionally, the susceptibility of the SiC fibers to corrosion reactions, e.g., oxidation, are affected by the surface composition of the fibers. A variable (or incorrect) surface composition would therefore, result in an attack on some (or all) fibers, thereby leading to a poor product.
One way to solve the processing problem is to use additives or to change the process conditions for slip casting. However, in order to change the conditions, experimentation is required to develop new conditions. This means that every time a different source of fibers is used or a new batch of fibers from the same source is used, new conditions must be determined. Because this experimentation is very time consuming, it would disrupt the manufacturing process. Therefore, there is a need for a process to deposit a coating on said fibers which will give a consistent surface composition. It is desirable to be able to adjust the surface composition and chemistry over a range. A single type of coating may be desirable for one application, while another surface composition and chemistry may be appropriate for another application. Our invention allows one to systematically control the surface composition of the fibers by appropriate selection of particular silane coating agents and processing conditions.
The prior art methods of coating fibers are vastly different from the process of the present invention. For example, U.S. Pat. No. 4,131,697 discloses a method of coating carbon filaments with silicon carbide. This method involves passing the carbon filament into a first heated reactor (at 1100-1200.degree. C.) containing silicon tetrachloride and hydrogen, followed by passing said filament into a second heated reactor containing methyltrichlorosilane and hydrogen thereby forming a silicon carbide coating of at least ten microns onthe filament.
Similarly, U.S. Pat. 4,373,006 describes an electrically nonconductive fiber comprising a carbon fiber coated with silicon carbide. Again the carbon fibers were coatd with silicon carbide by passing the carbon fibers through a heated chamber (1100.degree.-1200.degree. C.) containing methyldichlorosilane plus methane or hydrogen.
In marked distinction, the process of the present invention comprises contacting fibers with an organochlorosilane and heating at a temperature of about 350.degree. C. in an air or a non-oxidizing atmosphere (depending on the desired final composition) to form a coating having the formula SiC.sub.x O.sub.y on said fiber. A preferred method of coating the fibers is to impregnate the silicon carbide fibers with a solution of the organochlorosilane, drying the impregnated fibers and then heating as described above. The instant invention differs in several ways from the prior art. First, the fibers are not heated to extremely high temperatures (350.degree. C. versus 1,000.degree. C.). Second, only an organochlorosilane is used to form the coating. That is, the instant invention does not react an organochlorosilane with methane or hydrogen as described in the prior art.
A third distinction between the instant invention and the prior art is that applicants have discovered that the composition of the coating can be carefully controlled by choosing the appropriate organochlorosilane and by controlling the atmosphere during heating. The prior art discloses that the coating may be either SiC or SiO.sub.2 (see U.S. Pat. No. 4,373,006, column 2, lines 18-20). In contrast to this the coating of the instant invention has the empirical formula SiC.sub.x O.sub.y, where x ranges from about 0.4 to about 2.7 and y ranges from about 0.4 to about 3.2. Thus, virtually an infinite number of compositions are possible using the instant invention, whereas only two compositions are possible using the process of the prior art.
The instant invention provides a simple and effective method of applying a uniform coating to fibers such that the surface composition of said fibers is controlled and always the same. Using our invention the surface composition may be chosen from a wide range of compositions. This means that regardless of the sourjce of the fibers, the instant invention provides fibers which have a desired surface compostion and surface chemstry. The practical effect of controlling the surface composition of fibers is that slip casting can be performed using one set of process conditions, composites can be fabricated with controlled interface, thereby yielding better mechanical properties, and oxidation resistance may be imparted to the fibers.